The present invention relates to a system for correcting video signals having a limited amplitude range, wherein the correction signal is to be combined with the video signals to be corrected.
The present invention relates specifically to aperture correction circuits wherein errors arising both on the transmitting and the receiving side are to be eliminated or greatly reduced. A standard aperture correction of video signals generally comprises a correction for errors arising from distortion of the video signals as a result of the finite size and lack of uniformity in the impingement of the scanning dot both on the signal electrode on the pick-up side as well as on the screen on the receiving side. In known aperture correction circuits, an optimum visual correction is carried out only for the portions of the video signal having a medium brightness. This results in an overdriving of the circuits in response to sudden amplitude changes in the video signal. This in turn results in a widening of the discontinuity and a resultant loss of picture quality. In order to eliminate these particular correction errors and others, it has been suggested that the correction signal should be modulated as a function of video signal amplitude prior to heterodyning with the video signal in such a manner that overdriving of the heterodyned signal into the blacker than black or whiter than white region is eliminated.
One known system of the above-described type is shown in FIG. 2 of U.S. Pat. No. 3,536,826. The system shown there is a system for vertical aperture correction. As becomes evident from the description of this known circuit, only the oscillations due to overdriving in the heterodyned video signal are limited. A substantial amount of circuitry is required to achieve this purpose. No correction takes place between the video signal amplitude levels of 0% (black) and 100% (white). Thus with the known circuit arrangement, the amplitude range of the video signal can be selected and the vertical correction signal is cut off at the limits of this signal.